Eddy current testing arrangements can be used for the testing of many types of parts but the testing of the entire surface of a part generally requires a substantial amount of time and for this reason, eddy current testing not always is satisfactory, especially in the testing of small cylindrical parts. Also, eddy current inspection does not readily adapt itself to automatic inspection.
Various arrangements have heretofore been proposed for magnetic testing of parts by effecting engagement between a surface of a part and a magnetic tape or sheet with the part being magnetized prior to or during such engagement, a magnetic flux pattern of the surface of the part being transferred to the magnetic tape or sheet. The flux pattern on the tape or sheet is then detected by scanning with a reproducing head or by applying magnetic particles over the surface of the tape or sheet.
Included in the prior art disclosures is that of the Kodis U.S. Pat. No. 2,648,435 wherein a magnetized cylindrical specimen is rolled between a portion of a magnetic inspection belt moving in one direction and a conveyor belt moving in an opposite direction moving at a greater speed, the magnetic pattern recorded on the inspection belt being detected by a magnetic head. Other patents disclose arrangements in which a tape is pressed against a part surface with the tape being moved by engagement with a moving part or with the part being stationary and the tape being carried and driven in a manner to move along or around the part. For example, the O'Connor et al U.S. Pat. No. 3,760,263 discloses an arrangement in which a magnetic tape is entrained for movement on a support structure with the support structure and an elongated object being tested being moved relative to one another about a longitudinal axis of the object, preferably with there being a generally helical path of movement of the area of interengagement between the tape and the object. Other examples are disclosed in the Nasir et al U.S. Pat. No. 3,262,053, the Crouch et al U.S. Pat. No. 3,341,771, the Mandula U.S. Pat. No. 3,593,120 and the Forster U.S. Pat. No. 3,656,054.
The arrangement of the O'Connor et al patent is highly advantageous in the testing of large parts and especially parts such as square billets but would not be generally satisfactory for the testing of roller bearings or other small parts. The arrangement of the Kodis patent poses serious problems especially with respect to accurately controlling movement of the specimen between the inspection and conveyor belts and proper engagement of the part with the inspection belt. The other prior patents also have serious limitations with respect to the applicability to the testing of small parts.